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Chris Burgoyne - Research Projects
mechanics of bonded FRP plate failure - Active
||Carbon Fibre Plates can be bonded to the
tension faces of concrete structures to provide additional strength. Failure
often takes place in the concrete layer just below the adhesive, either
by peeling at the ends or by a crack propagating from an existing shear
crack in the concrete. The conventional methods of analysis use finite
elements to model the CFRP, the concrete and the adhesive. But the exact
shape of the adhesive layer is unknown, and there is a singularity in the
stress concentration at the interface. Furthermore, the condition of the
interface is unknown, and unknowable - it is impossible to say if there
are small voids in the adhesive and the exact surface texture cannot be
known. Our analysis applies fracture mechanics principles - we assume that
a crack is present and the method asks whether a propagation of the crack
requires additional energy (in which case it will only propagate slowly
as the load increases), or releases energy (in which case it will fail
testing of aramid yarns - Active
||The stress-rupture performance of aramid
fibres governs the allowable stresses that can be applied to them when
used as prestressing tendons for concrete. Such tendons would have design
lives of 120 years, so we clearly cannot wait that long to obtain the relevant
test data. This project, undertaken by by Nadun Alwis, has evaluated various
strategies for using accelerated testing. It has been concluded that the
Stepped Isothermal Method can be used to produce test results in reasonable
time scales. The method can also produce master curves for the creep response
of the fibres.
Imaging of Concrete Structures - Active
||In a joint research project with the Herchel
Smith Laboratory for Medicinal Chemistry, the techniques have been determined
to enable the study of the fracture state of hardened concrete samples.
The technique is non-destructive, which enables multiple scans to be made
of the sample at different loading stages. It has been shown that the internal
structure of concrete can be determined in the first 10 hours after casting,
and fractures can be studied in wet samples. If registration markers are
present, the two images can be combined to show how the fractures relate
to the aggregate distribution. At the moment the method is useful for the
study of the meso-structure of small concrete samples, but it is intended
to extend the work to larger samples. For
more details ...
wheels as prestressed structures - Active
||There were three inventions that gave
us the bicycle wheel as we know it today - each is associated with an application
of prestressing to a structural system. The first and most obvious is the
tensioned spokes - the rider's weight is carried from the forks to the
ground not by hanging off the top spokes, but by reducing the pretension
in the lower spokes - only a couple of spokes are carrying the load at
any one time. The second is the pneumatic tyre, where the compressive load
is carried to the ground by reducing the tension in the sidewall. The air
pressure in the tyre does not change when the load is applied. The final
prestressing system is the tyre cord, which is shorter than the perimeter
of the rim. The cord is thus in tension, holding the tyre on the rim, which
enables the pretension in the sidewalls to be reacted. A
paper describes these systems and the tests that were carried out.
of Cantilevered Stone Staircases- Active
||There are many 18th and 19th century buildings
which cantilevered stone staircases. They are made of brittle stone of
unknown quality; they are often on means of escape and sometimes in public
buildings, but the mechanism by which they carry load is uncertain. There
is a widespread belief that they work by carrying forces down the flight
to the base, with the wall providing only torsional restraint, but some
flights are very long and there are some which spiral down many storeys.
There are also horizontal cantilevered landings which cannot behave in
this way. Engineers are being asked to sign certificates to say that these
staircases are "safe" which means putting their PII on the line. Our study,
in conjunction with Price and Myers has involved tests on stine in torsion
and also dynamic tests on staircases and landings in a Cambridge college.
growth in response to stress - Active
||It is known that bones grow in response
to stress and decay when not used. They are also known to be near-optimal
structures with relatively low factors of safety by normal structural standards.
Athletes have more bone mass in the limbs they use most (e.g. the serving
arm of tennis players) and people with broken legs need to exercise to
prevent loss of bone structure. But what are the rules that govern this
behaviour? Biology must reflect structural mechanics - our research seeks
the rules that have to be adopted to allow bone growth to be stable with
the minimum requirement for genetic coding. The images show sections obtained
by Saumarez showing various rib cross-sections - he showed that they were
optimal for the loadings to which they were subjected.
Algebra applied to beam stability problems - Active
||To determine the behaviour of beams and
columns which buckle an accurate description of the 3D buckled state is
required. If the pre-buckling deformations are significant the post-buckling
rotations are not adequately expressed as vectors and a more accurate analysis
is required. Direction cosines, Euler angles and finite rotations
have all been tried, but have varying problems. With Joan Lasenby
of the Speech Vision and Robotics group we have been applying the principles
of geometric algebra to these problems. Geometric algebra is a consistent
method of specifying orientation in multiple dimensions and is based on
Clifford Algebra. We have shown that these techniques can be aplied
successfully to these problems.
development of prestressed concrete - Active
||Boutiron Bridge near Vichy is probably
the most important structure in the development of prestressed concrete,
although it is a reinforced concrete arch. It was here that Freyssinet
realised the importance of creep, and this led to the invention of the
first practical system for prestressing. A few prestressed concrete bridges
were built during World War II, and development took off after the war
when the shortage of steel caused major problems. Very few details exist
about these bridges and many are not recorded. An on-going project is to
build up a database of these structures so that their condition can be
assessed. For more details ...
Ceramics - Active
||Ceramics offer a number of advantages
for many applications, but they are available in limited sizes, since they
need to be fired, and they are brittle, so they need to be expensively
toughened. Or do they? The photo on the left shows bathroom tiles, laid
end to end, and containing a pretensioning wire. The joints are made with
conventional tile adhesive from a DIY shop. The beam is then loaded in
4-point bending. Even in this poor photograph, the cracks on the tension
face are clearly visible, but the fracture does not propagate through the
beam. Even under this extreme loading, the beam can be unloaded with no
residual deflection. Brittle?
Material for Use in School- Complete
||In association with the Educational Trust
of the Institution of Structural Engineers, we are producing teaching material
for use in schools. A study carried out for the trust showed that
the biggest drop-off in the numbers studying the physical sciences occurs
immediately after GCSE level. It was recognised that many school
teachers were not sufficiently familiar with engineering, but would be
willing to use material if it were available. Thus the objective
is to show pupils in their final GCSE year how relevant physics is to modern
life; we have produced three modules with the general theme of "Stadium
Roof Design", which takes pupils through a Design Decisions Activity (aimed
at DT Resistant Materials Syllabus), Emirates Stadium Structural Analysis
(aimed at Mathematics and Physics - Forces and Motion), and Cantilever
Roof Design (covering Physics, Maths and DT material). Other modules
are in preparation. The material
can be freely downloaded and is intended for use by the teacher
without additional help.
of Palau Bridge- Complete
||The Koror-Babelthuap Bridge in Palau collapsed
on 26th September 1996, at around 5:45 in the afternoon, about 90 days
after work had been carried out to eliminate excessive deflection due to
creep during the 19 years since it had been built. The collapse was catastrophic,
killing two people and injuring four more, and occurred under virtually
no traffic load during benign weather conditions. Services passing through
the bridge between the country's two most populated islands were severed;
this caused the government to declare a state of national emergency and
request international aid for the thousands of people left without fresh
water or electricity. Why did a major structure collapse so soon after
engineering intervention. Was the repair at fault or was the original construction
at fault? A study has been carried out to investigate the various possibilities,
and the conclusions, which have been published,
Stability of Precast Concrete Beams - Complete
||Concrete beams are normally thought to
be not susceptible to buckling, since the sections are much chunkier than
steel sections. But the desire to increase the span of concrete bridges,
and still to allow the transportation of the beams around the country,
has led to the development of longer, deeper, narrower and heavier beams,
and checks now need to be taken for stability. It has been shown that the
beams are most susceptible to buckling when the beams are being lifted
into place, since the beam can rotate and buckle about its minor axis without
causing torsion. The beams also need to be checked when they are placed
in position, before the top slab has been cast. This work has been undertaken
by Chris Burgoyne and Tim Stratford, now at the University of Edinburgh.
Viability of FRP Prestressed Concrete Structures - Complete
||Fibre Reinforced Polymers and aramid ropes
have been identified as suitable materials for reinforcing and prestressing
concrete. Much research has been carried out over the last 20 years, but
the take-up by industry has been very slow. The problem appears to be economic,
but this topic is rarely addressed by researchers, who concentrate on the
purely technical issues. This project, undertaken by Iannis Balafas, looks
at the optimal design constraints, the rates of corrosion in structures
reinforced with steel, and traffic delay costs. It has been shown that,
if whole-life costing and realistic discount rates are used, it is worthwhile
spending a little more on non-corroding materials when structures are first
built. The image on the left shows part of a design chart for finding the
of Concrete Beams - Complete
||The increased use of FRP reinforcement
has raised technical questions regarding the behaviour of the concrete.
Because FRPs have no ductiility themselves we must ensure that the complete
section shows some ductility and gives warning of failure. This can partly
be achieved by the cracking behaviour of the concrete, but we must ensure
that the concrete srushes before the FRP snaps, and that as much energy
as possible is absorbed in the compression zone. Slightly paradoxically,
development of new tension materials focusses attention on the failure
mechanisms of concrete. In this work we are studying how concrete failures
are localised. This image shows strain distributions (measured optically).
Two large cracks are visible, as is the localised failure region at the
top of the right-hand crack.
Reality Image of Prestressed Concrete Bridge Construction - Complete
||To demonstrate the internal profiles of
prestressing cables within a typical prestressed concrete bridge, a virtual
reality model of the Torridge Bridge at Bideford in Devon has been constructed.
The observer can fly over and through the bridge, and can make the concrete
transparent to reveal the internal cable profiles. The construction sequence
can also be followed. The work was carried out by a final year project
student, James Stevenson, with assistance from Parson Brinckerhoff who
supplied accurate bridge drawings. For
more details ...
|Effect of construction
sequence on creep of prestressed concrete structures - Complete
||When an indeterminate beam is constructed
in stages the bending moment due to the dead load differs from that which
would be appplicable if the beam was built in a single operation.
Because concrete creeps, the distribution of bending moments changes over
the next few months. Conventional wisdom says that the bending moment
diagram always lies somewhere between the "as-built" diagram and the "monolithic"
bending moment. It is then possible to check that the structure is
satisfactory under these two extremes. But is this always the case.
Are there combinations of constructions sequence and temperature which
can mean that this assumption is not safe?
of Continuous Prestressed Concrete Beams - Complete
||The design of continuous prestressed concrete
beams is complicated because the prestress sets up secondary moments. These
need to be known before the cable can be designed, but cannot be determined
until after the profile is known, which is paradoxical. Design of such
profiles can thus be a hit and miss affair; guessing a profile, calculating
secondary moments and checking if the profile is satisfactory. By developing
Low's theory of the "third equation" (Procs ICE, 73, 351-364, 1982),
a design procedure has been developed which leads sequentially through
design of the cross-section to selection of the prestressing force and
finally to determination of the cable profile. Very few assumptions need
to be made and the process does not need complicated calculations.
Systems for the Design of Prestressed Concrete - Complete
||Expert systems have been proposed for
many applications. The claim is that by allowing a "knowledge engineer"
to interview "domain experts", a system can be designed which will allow
a computer to be used to make the decisions for itself. Quality is judged
by the number of rules the system contains. Our conjecture is the opposite
of that; we believe that the computer should be used to calculate
what it can, using logic programming only where decisions have to
be made. Quality in our expert system is measured by how few rules the
system includes. We have shown that such a system can be applied to the
design of continuous prestressed concrete viaducts, using earlier work
on the design of the cable profiles. The picture shows Grangetown
Viaduct in Cardiff, designed by Benaim. It was not designed by our
software but is the sort of structure we have analysed.
|FRP spirals to enhance
compression behaviour of concrete - Complete
||FRP reinforcement used as tension elements
in beams are brittle, which implies that the concrete in the compression
zone must be made more ductile to avoid sudden failure. One way of
achieving this is by providing confining spirals of aramid reinforcement
to keep the concrete in a state of triaxial compression. If this
sort of reinforcment was used in the compression zone, which is rectangular,
there would be three types of confinement - the normal zone, which is confined
by a single spiral; a doubly confined zone where spirals overlap, and an
area in the cover outside the spiral which is unconfined. What are
the relevant governing equations? The image shows a block with all
three types of confinement. Final failure occurs when the AFRP yarns
failed in tension.
joint mechanics - Complete
||The articulation of the elbow joint is
complex. The forearm can be flexed relative to the upper arm, and also
twisted. One can also carry significant loads in many of these orientations.
Designing a mechanical engineering joint with the same freedoms is extremely
complex, as robot designers have found; the image on the right shows a
crude (and unworkable) representation as a mechanical system. The aim of
this project is to determine the shapes of the mating surfaces and the
positions that the bones take up as the various degrees of freedom are
exercised. This involves structural engineering (in the load paths), mechanics
(following the motion), computer vision (to determine the shapes of the
mating surfaces) and computer modelling (to put all the components together).
It thus requires an understanding of many aspects of engineering, as well
bonding of FRP tendons - Complete
||This project was undertaken with Janet
Lees. It was based on a paper presented
at Vancouver in 1993. This questioned the validity of trying
to improve the bond of FRPs to concrete, since this can lead to premature
snapping of the tendon. Instead, we showed that it is better to partially
bond the tendons to the concrete, which allows controlled movement.
It is possible to arrange matters so that the beam is almost as strong
as a fully-bonded beam, and has the rotation capacity of a beam with unbonded
tendons, as shown in the figure on the left. Various papers have
been published on this work. Experiments;
Bond Behaviour; Analysis.
of barrel and spike terminations for parallel-lay aramid ropes -
||Parallel-lay aramid ropes can be anchored
by means of a barrel-and-spike termination, which grips the aramid fibres
in an annular ring. Many tests have been carried out to determine the optimum
shape and length of the barrel and spike, in association with Linear
Composites Ltd. The exact shape of the spike is commercially confidential.
Finite element analyses have been performed to model the stress-state as
the spike, fibre and barrel are brought into contact and loaded until the
fitting is secure. With terminations at both ends of a rope in a tension
test, the rope normally breaks away from the termination.
variability and bundle effects in parallel-lay ropes - Complete
||Fibres used in ropes are brittle.
This makes them very susceptible to variability, since if one yarn is weak
it will snap, shedding load to its neighbours. The strength of an
assembly is thus less than the sum of the strength of the whole.
This has major impacts on the conversion efficiency that can be achieved.
The work is complicated; quality control in yarn manufactire is important,
as is careful assembly of the rope. Additional slack introduced in
assmebly effectively makes the yarns more variable, which in turn reduces
the strength even further. There are also important length effects
- in long ropes a broken yarn can pick up load by friction from its neighbours.
The rope can thus be seen as a chain of variable elements, and a weakest
link theory applies. Many papers
have been published with Amaniampong, Mills and Flory.
block behaviour of concrete structures - Complete
||The behaviour of prestressed concrete
beams in the vicinity of the anchorage has been investigated by looking
at the behaviour of reinforced concrete blocks under patch loads applied
to their ends. Both upper- and lower-bound plasticity models have
been used to compare with these results. As a result we now have
better understanding of the behaviour of the concrete in this region.
This work was undertaken by Tim
Ibell, now at Bath University and Chris
has a web page describing the importance of the work.
Three papers have been published; Experimental;
plasticity analysis; lower
cement couplers - Complete
||Gripping FRP rods with conventional wedges
causes stress-concentrations which can lead to premature failure of the
rods. By using expansive cements in an annular region between the
FRP rod and an external steel tube, the FRP can be anchored without stress-concentrations,
which allows the full strength of the rod to be achieved. Once anchored,
the rod can be linked to an external loading system, or coupled to another
rod. This work is a development of that carried out by Harada at
the University of Nagasaki in Japan. A paper
describing the method has been published.
|Behaviour of concrete
beams with FRP shear reinforcement - Complete
||Fibre Reinforced Plastics can only really
be useful if all the steel in a section is replaced. Thus, the shear
links have to be replaced as well as the flexural steel. But our
techniques for analysing beams with steel reinforcment rely on the steel
yielding so that we can apply plasticity theory. This is the fundamental
assumption that underlies the truss analogy, although it is rarely stated.
There are many proposals for using truss models for FRPs, which allow for
the brittle FRPs by limiting the strain to those of elastic steel.
This ignores the fact that the steel must be yielding to allow the truss
model to work. A revised analysis, based on elastic distributions,
has been undertaken by Tim Stratford (now at the University of Edinburgh)
to show that this can lead to unsafe predictions for the strength of beams
Reinforcing Bars - Complete
||Very highly stressed pretensioning wire
can be cast centrally into long concrete prisms, which can then be cut
into shorter lengths and used as reinforcing bars. This could be a useful
of using FRP bars with very high strain capacities, without the need for
prestressing on site, since the pretensioning operation could be acrried
out at specialist precast factories. There are some limits imposed by the
need to prevent the pretenioned rod splitting, and bond must also be assured
between the prism and the concrete, but tests have shown that the full
strength of pretensioning wire can be utilised at strains normally seen
in reinforced concrete structures. This project was undertaken as a final
year undergraduate project by Graham Redman.
Torsional stiffness of junctions - Complete
||To determine the St. Venant's Torsional
Stiffness of a cross-section, engineers often assume that the cross-section
is made up of a number of flat plates. That approach works well for steel
sections, where the flat elements are thin in comparison to their width,
but it does not work well for concrete sections where end effects are more
important. There are particular problems associated with T-junctions and
L-junctions, especially where the section has chamfers. A large proportion
of the torsional stiffness comes from the junction. In this work, the stiffness
of the junction is calculated separately, which can then be added to that
of the adjacent flat plates. The error compared with exact calculations
is much reduced, but the method can still be applied by hand calculation.
The method is described in a paper which can
be used as a design manual.
Ilyushin Yield Surface - Complete
||The Ilyushin Yield surface is frequently
used to determine the full plasticity surface for steel plates loaded in
tension and bending. Ilyushin's original analysis was too complicated,
in the days before computers, so he proposed an approximation. His parametrisation
of the surface led to numerical ill-conditioning, so his original analysis
has never been used. However, we have shown that if different aparmtrisation
is used, the ill-conditioning can be eliminated, so that an exact formulation
can be used. The image shows the Ilyushin surface, in Q-space, as a continuous
curved surface - the approximation uses two flat surfaces, which cause
problems at the intersection, and does not give correct values.
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